What is Sound?

Examples of objects that produce sound include the strings on a guitar or violin, the reed in a wind instrument mouthpiece, a trumpet player’s lips, and the head on a drum. All of these sources have one thing in common – they vibrate, creating variations in air pressure, called sound waves. Sound does also travel through other mediums, such as water and solid objects – but seeing as air is the medium that usually surrounds us, we’ll concentrate on that!

The image below shows s a simplified, illustrative picture of a vibrating guitar string. The string is anchored at both ends, and stretched so it is taut. When it is plucked, bowed, or struck, it is set in motion and vibrates. During this motion it moves from its point of rest (the position it naturally returns to when not in motion), labeled A, and out to an extreme, labeled B. As it approaches B, the tension in the string increases until it is not able to move any further, and it rebounds back in the opposite direction, through A to the opposite extreme, C. Tension builds as it moves towards C, and causes the string to reverse its direction again, so it moves back through A towards B. A little energy is lost with each consecutive change in direction, so the string gradually moves less and less (getting quieter and quieter) until it is stationary and silent back at its point of rest, A.

The motion of a vibrating string, and a resulting sound wave.

As the string moves from A to B, it squashes the air molecules to the left of the string closer together. This increases the air pressure in that spot, causing a compression. This compression then travels outwards from this source at the speed of sound. The air molecules themselves do not move far – the compressed group of molecules bump into the adjacent molecules, which bump into the next set, passing the compression on.

As the compression travels outwards, the string moves back through A, where normal atmospheric air pressure is restored. During the subsequent motion from A to C, the air molecules to the left of the string are drawn further apart, to fill the space where the string used to be. This causes a decrease in air pressure adjacent to the left of the string, called a rarefaction. This rarefaction travels outwards, following the previous compression. As the string returns to A, normal air pressure is once again restored behind the rarefaction. This continuing motion propagates the alternating compressions and rarefactions of sound waves behind each other. Greater motion and displacements from the point of rest create greater variations in air pressure and louder sounds.

If we analyzed the resulting sound wave from the right of the string in the diagram, the compressions and rarefactions would be reversed.

The above image also shows two ways of graphically representing these sound waves. The first is a series of blobs representing the relative spacing of the air molecules – the air pressure. The second is a time/amplitude graph. This graph may look familiar. It is the shape of a sine wave – a pure tone made up of one single frequency. An actual vibrating string produces a much more complex harmonic waveform than a sine wave, shown below.

A loudspeaker functions in a similar way. A speaker cone, or driver, is set in motion – it pushes outwards and sucks inwards, propagating compressions and rarefactions in front of it, which then travel towards the listener.

The waveform produced by an actual guitar string, showing the overall motion of the string produced by the interaction of all the frequencies present.

Time/amplitude graphs are based on measurements taken at a single point in space, over a duration of time. This is similar in principle to what a microphone does – a mic is positioned at a single point in space and takes measurements of air pressure over time, capturing frequency and amplitude information.

Dr. Ian Corbett is the coordinator of the Audio Engineering Program, and Professor of Music Technology and Audio Engineering at Kansas City Kansas Community College. He also owns and operates “off-beat-open-hats – recording and sound reinforcement” which specializes in servicing the needs of jazz and classical ensembles in the Kansas City area. Since 2004, he has been a member of the Audio Engineering Society’s Education Committee, and has mentored, presented, and served on panels at local, regional, national, and international AES and other professional events. Ian has also authored articles on audio recording related subjects for Sound On Sound magazine.

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